1. Field of the Invention
The present invention relates to processors, and more particularly to improving the dynamic range of analog to digital converters (“ADC”).
2. Background
Advances in digital integrated circuit technology demand lower power supply voltages for increased performance and reduced power. Hence, the dynamic range available for the analog portions of mixed-signal integrated circuits (i.e., integrated circuits with both digital and analog circuits) has decreased accordingly. In general, the analog supply voltage cannot exceed the digital supply voltage in mixed-signal devices.
Typically, a central processing unit in a system using integrated circuits is available for calibrating voltages in common ADC applications. A prior art system illustrating the problem with conventional systems is shown in FIG. 1. Analog input signal(s) 102 (from Channel 1 through Channel N) enter ADC 101. A digital output 110 is sent to a central processing unit (CPU) 110 or any other component. A reference voltage 106B is applied to REFP 106, while REFN 108 is connected to analog ground.
Reference voltage 106B is typically implemented using a band gap reference (“BGR”) circuit 103 where a resistor or current source 104 (referred to herein as “Source 104”). The current provided by source 104 must remain within the operating range of BGR device 103.
To compensate for supply voltage variations, current sources are often preferred over resistors. Current source devices typically require about a one-volt differential between their power input (V+ connection 105) and their output to BGR 103.
To be measurable, analog input voltages must remain in a range defined by REFP−REFN and REFP 106 must have a value that is less than or equal to AVDD 107. Hence, the available dynamic range of ADC 101 is diminished by the voltage required to operate the current source (or resistor). In low voltage integrated circuit designs this loss can be significant. For instance, in a 2.5V power supply environment using a current source that requires 1V to operate represents a forty percent (40%) loss of dynamic range. Also, BGR device(s) 103 is commonly available only in 2.5V and 1.2V versions. In a 2.5V power supply environment, a 1.2V BGR device 103 would have to be used in order to provide the operating voltage for the current source or resistor giving a 52% loss of dynamic range.
Also, if AVDD is measured in a conventional implementation, it must be divided down to a value less than REFP 106, as shown in FIG. 2 (using resistors 111 and 112), before being applied to an analog input. This reduces the accuracy of the measurement.
Therefore, what is desired is a system that can efficiently increase the dynamic range of ADCs without increasing the power supply voltage.